Method of making a sheet of building material

ABSTRACT

A method of making a sheet of building material, useful for roofing or siding applications, and the sheet material made by such method. A mixture of a thermoplastic resin and mineral filler is prepared, the resin comprising about 10% to 40% by weight of the mixture and the filler comprising about 60% to 90% by weight. The mixture is mixed at a temperature above the melting point of the resin and is formed into a sheet at such temperature. The sheet is allowed to cool until the surface is at a temperature below the midpoint of the melting range of the thermoplastic resin, e.g. in the range of 205 to 225 degrees F. for polyethylene, at which point it is strained by passing it through calendering rollers. The sheet material made by this process, having a high concentration of mineral filler, is fire resistant, durable, ductile, of moderate weight and resistant to weathering. It can be produced using relatively inexpensive manufacturing equipment. The sheet material can incorporate a high proportion of recycled resin, such as recycled high density polyethylene.

FIELD OF THE INVENTION

The invention pertains to building materials in sheet form forapplications such as roofing and siding. More particularly, it pertainsto sheet materials comprising thermoplastic resin and mineral filler andto methods of making such materials.

BACKGROUND OF THE INVENTION

Many types of natural and synthetic roofing materials are available inthe market. Some of the more popular natural types include naturalslate, shakes and shingles. Natural slate has long been a popularroofing material due to its attractive appearance and durability andalso because it possesses other highly desirable properties such asbeing fireproof and waterproof. It is, however, very expensive and as aresult is normally used for roofing in only the most expensive housesand in other structures where the increased cost can be justified. Slateis a brittle material and can be cracked or broken rather easily.Natural slate tiles are quite durable; however they require asubstantial amount of labor in their installation and can break onimpact. They are inherently fragile and suffer much breakage duringshipping and installation. They are fragile even after installation onthe roof and can be damaged by foot traffic on the roof. Slate tilestend to be excessively heavy and dangerous in earthquakes and highwinds, and will fall through the roof in the event of a fire. Since thetiles are so heavy, they are also expensive to ship. Also, due to theweight of natural slate, extra structural support is required for slateroofs compared to cedar shake or shingle roofs or asphalt roofs.

Wood shakes and shingles are subject to breakage, rot and loss ofcoloration. Their cost is relatively high and they are labor-intensiveto install. Furthermore, wood shakes and shingles can be relativelyheavy and are flammable, porous and cannot withstand relatively highwind velocity. A disadvantage of wooden shakes and shingles is that theyabsorb moisture and swell. Therefore, they must be applied in aspaced-apart arrangement to allow room for moisture expansion. Becauseof the propensity of wooden shakes and shingles to absorb water, withtime they tend to curl and not remain flat on the roof.

Synthetic roofing materials provide some advantages over these naturalmaterials. They are moldable and light in weight. However, they havenot, in general, been fully acceptable in terms of performance becausethey often do not meet all the requirements desired for roofingapplications. As an example, synthetic roofing materials typically havehigh concentrations of plastic or rubber content in the formulation thatdirectly effects the fire resistance of the products because plastic andrubber materials lack fire resistant properties. In some case, fireresistance of the product has been enhanced by adding a highconcentration of flame retardant, which in turn, makes the product muchmore expensive. Products with recycled rubber may also have a strongodor on warm days due to the gassing off of volatile components.

One desirable property of any synthetic roofing material is to be ableto resist fires. This is particularly true in regions having a hot anddry climate, although fire resistance is desirable everywhere. Aparticularly important aspect of fire resistance is the ability of theroofing material to prevent the spread of fire from a source of heat,such as a burning ember, from burning through the roof to thereby exposethe roof deck or interior of the building to a fire.

Another desirable property of any roofing material is that it haslong-term ductility, enabling installers and owners to walk on the roofat any time during the roofing product's lifespan, without causingdamage. Ductility in roofing materials also allows repairs to be moreeasily undertaken in the event of damage, such as that incurred fromfalling tree limbs.

Additionally, with public awareness increasing about the importance ofrecycling to consume fewer materials, it is desirable that recycledmaterials be used as a portion of any synthetic roofing materials. Thisprovides a market for recycled materials, and recycling practices areencouraged if there is a known commercial application for thesematerials.

There have been various attempts to meet the requirements of syntheticbuilding materials by means of molded products made from plastic resinsand inorganic fillers. The following patent documents are examples.

U.S. Pat. No. 5,571,868 (Datta et al.) discloses elastomeric polymercompositions used in sheet materials for roofing. The materials cancomprise elastomeric polymers and inorganic fillers, and can be made byheating, roll milling and calendering the mixture.

U.S. Pat. No. 3,070,577 (Gessler et al.) discloses polymeric compositescontaining inorganic fillers. The mixture may be formed into thinsheets, useful for roofing, by heating it and passing it throughrollers.

U.S. Pat. No. 4,263,186 (Bluemel) discloses a thermoplastic compositemade by mixing polyethylene and calcium carbonate. It may be formed intoa sheet on a rolling mill and the resulting material used in buildingconstruction.

US 2005/0140041 A1 (Seth) discloses a synthetic building material madeby extruding a mixture of plastic resin, which may be recycledpolyethylene, and a filler, which may be limestone.

U.S. Pat. No. 3,976,612 (Kaji et al.) discloses a method of making acomposite sheet by mixing an inorganic calcium compound withpolyethyelene, kneading and heating the mixture into a paste andcalendering it into a film.

GB 1 534 128, published Nov. 29, 1978, discloses a method of making acomposite sheet comprising polymer resins and calcium carbonate byheating the composition, rolling it into a rough film at a temperatureof 160 to 180 degrees C. through a roller nip of 2-5 mm and rolling therough film into a smooth film at a temperature of 160 to 180 degrees C.through a roller nip of 0.1 to 1 mm.

However, there remains a need in the building industry for a syntheticsheet material which has the characteristics that are important for usein roofing and siding applications. It would be desirable to have asynthetic material that could overcome the disadvantages of the previousattempts to produce such materials. There is a need for a syntheticsheet material that is durable, moderate in weight, inexpensive, fireresistant, tough and ductile, that can be made in part by using recycledmaterials, and that can be produced using relatively inexpensiveproduction equipment.

SUMMARY OF THE INVENTION

The present inventors have discovered that, when a highly mineral-filledthermoplastic composite is strained under controlled temperatureconditions, it is possible to reduce the loss of toughness and ductilitythat is normally associated with the use of high filler ratios, andproduce a sheet material having enhanced toughness and ductility.

The invention provides a method of making a sheet of building material,useful for roofing or siding applications, and the sheet material madeby such method.

A mixture of a thermoplastic resin and mineral filler is prepared, theresin comprising about 10% to 40% by weight of the mixture and thefiller comprising about 60% to 90% by weight. The mixture is mixed at atemperature above the melting range of the resin and is formed into asheet at such temperature. The sheet is allowed to cool until thesurface is at a temperature below the midpoint of the melting range ofthe resin, at which point it is rolled (calendered), for example bypassing it through calendering rollers.

According to one embodiment of the method, the surface of the sheet isallowed to cool to a temperature in the range of 205 to 225 F. (96 to107 degrees C.) for rolling. In another embodiment the resin ispolyethylene and the surface temperature range is 210 to 220 degrees F.(99 to 104 degrees C.) In yet another embodiment, the resin ispolypropylene and the surface temperature range is 220 to 250 degrees F.(104 to 121 degrees C.)

The sheet material made by this method, having a high concentration ofmineral filler, is fire resistant and is less expensive than productsmade with a higher proportion of resin. The sheet material is durable,ductile, of moderate weight and resistant to weathering. It can beproduced using relatively inexpensive manufacturing equipment. The sheetmaterial can incorporate a high proportion of recycled resin, such asrecycled high density polyethylene (HDPE).

In prior art processes for making sheet materials comprisingpolyethylene and mineral filler, the amount of HDPE, and especiallyrecycled HDPE, that can be incorporated in the product is limited due tothe brittleness caused by HDPE. The method of the invention reduces thebrittleness of the sheet material and permits the use of higher levelsof HDPE, and especially recycled HDPE, than in prior art processes.

These and other advantages and features of the invention will beapparent from the following description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the effect of straining at 220 degrees F. andat room temperature.

FIG. 2 is a graph showing the effect of increasing levels of filler withrecycled HDPE.

FIG. 3 is a graph showing the effect of increasing levels of filler inrecycled polyethylene/limestone composite that is strained at 220degrees F.

FIG. 4 is a graph showing the effect of straining at varioustemperatures.

FIG. 5 is a graph showing the effect of mica.

FIG. 6 is a graph showing the effect of compressive and tensileproperties of strained and unstrained specimens.

FIG. 7 is a graph showing the effect of straining composite composedwith polypropylene resin.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The method of the invention produces a sheet of building material, suchas a roofing panel or siding panel, comprising a thermoplastic resin andmineral filler. In this specification, the word “sheet” includes a piecehaving any thickness that is practical for use in building applications.In the preferred embodiment, the resin is polyethylene and the filler islimestone (calcium carbonate) powder.

The limestone is heated to between 300 and 350 degrees F. (149 and 177degrees C.) and the polyethylene is added to it, in pellet, flake orpowdered form. Additives of the types discussed below are also added.The temperature of the mixture is maintained above the melting range ofthe polyethylene. It is preferably maintained at about 300 to 325degrees F. (149 to 163 degrees C.) for polyethylene-limestoneformulations. This temperature is maintained while the polyethylene andlimestone are mixed. Mixing is done by means of kneading in a twin-bladeor twin-screw blender in an open atmosphere. The open atmosphere allowsgassing off, i.e. the release of steam formed by moisture in thecomponents, to minimize entrapped gas and reduce the size and number ofvoids in the finished product.

The kneaded paste is formed into a sheet of the desired width andlength. The thickness is about one-quarter inch (6.4 mm), which issuitable for typical roofing and siding applications. The surfaces ofthe sheet are then allowed to cool to a temperature in the range of210-220 degrees F. (99-104 degrees C.), at which point the sheet ispassed between pressure-controlled calendering rollers which cause astrain in the sheet longitudinally of about 1% to 10%. The calenderingrollers simultaneously strain and cool the sheet, producing a finishedsheet having the desired physical properties. The calendering force usedto effect the desired degree of straining is up to about 15,000 pounds.The calendering rollers are maintained at a temperature below thesolidus temperatures of the polymers used in the mixture, for examplebetween room temperature and 150 degrees C. The rollers may bewater-cooled to maintain them in this range.

When the surface temperature of the surface of the one-quarter inchthick sheet is in the range of 210-220 degrees F. (99-104 degrees C.)the temperature in the middle of the sheet is in the range of 225-245degrees F. (107-118 degrees C.). For practical purposes in controllingthe process, the surface temperature is easier to measure.

The calendering rollers may be patterned to simultaneously embosssurface textures or features on the sheet during the straining step.Such surface features may include those resembling, for example, naturalslate, cedar shingles, cedar siding, wood grain or other artificialtexture or pattern.

By varying the force applied by the calendering rollers during thestraining step, the colour tone of the final product can be varied tobetter resemble the variation in colour tones found in natural productssuch as slate and cedar. The variation in force is created by randomlyvarying the pressure delivered to the actuators maintaining thecalendering force. The actuators are of a type with little or nostiction so that even small pressure changes effect a proportionalchange in calendering force.

A protective coating such as Varathane (trademark) polyurethanes orother polymeric coating can be applied to the surface of the producedsheet material. This significantly reduces the susceptibility of thesheet material to surface abrasion.

In the resin-filler mixture, the resin may comprise about 10-40% byweight of the mixture, preferably 10-35%, and more preferably about 20%;and the mineral filler may comprise about 60-90%, preferably 65-90%, andmore preferably about 80%.

The resin may comprise polyethylene. The polyethylene may comprise amixture of high density polyethylene and linear low densitypolyethylene. The ratio of HDPE to LLDPE may be about 3:1 by weight. Anyor all of the resin may comprise recycled material and it preferablycomprises at least half recycled material. The high density polyethylenemay comprise recycled milk bottles. Alternatively, the resin maycomprise polypropylene and/or recycled polypropylene.

The mineral filler is preferably limestone. Examples of other mineralfillers that may be used in the invention include dolomite, talc, silicaand flyash. The particle size of the mineral filler may be about 100mesh. Alternatively, a mixture of 100 mesh and 200 mesh may be used,e.g. about 90% of 100 mesh and 10% of 200 mesh, to marginally reduce thesusceptibility of the sheet material to surface abrasion. It has beenfound that limestone of larger particle size, e.g. 30 to 40 mesh orlarger, tends to result in a brittle sheet material.

The mixture of resin and mineral may also include additives that areuseful for particular applications. Where the sheet is to be used forroofing panels, a stabilizer such as carbon black (about 1-2% by weightrelative to the resin) may be added to the mixture to stabilize thefinished product against the depolymerizing effect of ultraviolet lightand sunlight. UV inhibitors may be included, such as Tinuvin 783 andTinuvin 328 (trademarks) made by Ciba Specialty Chemicals, and HostavinN321 and Hostavin ARO 8 (trademarks) made by Clariant. Inorganicpigments may be included, such as chromium oxide green, raw titanium,titanium white, and iron oxide such as raw sienna and burnt sienna.Licocene (trademark), a wax made by Clariant, may be included to enhanceductility.

The mixture may include mica, which has the effect of enhancing theductility of the sheet material and the effect of pigments. The mixturemay comprise about 0.1 to 1% by weight mica.

The sheet material of the invention is relatively fire resistant andfireproof. If increased fire resistance is desired, a fireproofingadditive may be included in the resin-filler mixture. Examples includehighly chlorinated naphthalenes, phosphates, organic fluorides,siloxanes and silicates.

The mixture may also include a processing stabilizer or lubricant suchas a metallic stearate, hydrocarbon, fatty acid, ester, amide,fluoropolymer, silicone or boron nitride.

Example 1

A composition was made comprising about 80% by weight limestone of 100mesh, 18% by weight polyethylene (13.5% by weight recycled HDPE flakesand 4.5% by weight virgin LLDPE) and 2% by weight of lubricants (zincstearate or stearic acid). The mixture was made, kneaded and formed intoa sheet at about 300-325 degrees F. (149-163 degrees C.). Sample #1 ofthis preparation was allowed to cool to a surface temperature of about210-220 degrees F. (99-104 degrees C.) and was strained at thattemperature by calendering rollers. Sample #2 of this preparation wasallowed to cool to ambient temperature of about 70 degrees F. (21degrees C.) and was strained at that temperature by calendering rollers.Sample #3 of this preparation was simply allowed to cool to ambienttemperature and was not strained.

The samples were tested for brittleness and ductility as follows.Samples of about 4.5 inches (11.4 cm) in length, 1 inch (2.54 cm) inwidth and 0.25 inches (6.4 mm) in thickness were loaded into athree-point flexural testing apparatus with end supports 4 inches (10.2cm) apart and with the load applied at the midpoint between thesupports. Loads were applied and the resulting displacement wasmeasured, giving results summarized in FIG. 1.

Sample 3 exhibited brittle behaviour and brittle fracture when tested inthe three-point flexural testing apparatus. Brittle behaviour isundesirable in roofing applications because simply walking on a brittleroofing material after it has been applied may cause fracture andpermanent damage. Additionally, brittle behaviour is not desired andductile behaviour is preferred when roofing materials are fastened intoposition with nails. Ductility when nailing reduces the probability ofthe roofing splitting or developing excessive internal stresses in theimmediate vicinity of the nail hole.

Samples 1 and 2 exhibited lower strength but significantly greaterductility than Sample 3. The ductility of Samples 1 and 2 is directlyrelated to the change in physical properties caused by allowing thesesamples to cool and then straining and cooling the samples. Sample 1exhibited increased strength compared to Sample 2; consequently Sample 1is considered the most desirable of the three samples for roofingapplications because of its ductility and moderate strength.

Example 2

The effect of having increased levels of limestone in sheet materialmade from a mixture of polyethylene and limestone was studied. Samples Ato D were prepared having the following compositions:

Sample A: 52% limestone, 46% polyethylene (of which three-quarters isrecycled HDPE and one-quarter is virgin LLDPE).

Sample B: 60% limestone, 38% polyethylene (of which three-quarters isrecycled HDPE and one-quarter is virgin LLDPE).

Sample C: 71% limestone, 27% polyethylene (of which three-quarters isrecycled HDPE and one-quarter is virgin LLDPE).

Sample D: 80% limestone, 18% polyethylene (of which three-quarters isrecycled HDPE and one-quarter is virgin LLDPE).

Each sample composition was kneaded at about 300-325 degrees F. (149-163degrees C.) and formed into a sheet. No straining of the sheets, as bycalendering, was carried out. The sample sheets (having dimensions thesame as in Example 1) were tested for brittleness and ductiblity using athree-point flexural test as described in Example 1. The results aresummarized in FIG. 2

Samples C and D fracture at low and moderate deformations while SamplesA and B did not fracture for the extent of the test. Samples A and Bdisplayed ductile behaviour that is useful in the construction industry.However, Samples A and B are comparatively expensive to produce becauseof the high thermoplastic resin and low mineral filler contents.

Example 3

The effect of straining at 220 degrees F. (104 degrees C.) of sheetsmade with different levels of limestone filler was studied. Samples A toD were prepared having the same compositions as in Example 2. They weremade and kneaded about 300-325 degrees F. (149-163 degrees C.) andformed into sheets. They were allowed to cool to a surface temperatureof about 220 degrees F. (104 degrees C.) and were strained at thattemperature by passing them through calendering rollers. The foursamples were tested for brittleness and ductility according to themethod described in Example 1. The results are summarized in FIG. 3.

The results show that all samples regardless of the level of mineralfiller exhibited ductile behavior.

Example 4

The effect of straining temperatures was studied. Samples were preparedhaving the composition described in Example 1. They were kneaded andformed into sheets at about 300-325 degrees F. (149-163 degrees C.).They were allowed to cool to respective surface temperatures of 195degrees F. (91 degrees C.), 200 degrees F. (93 degrees C.), 210 degreesF. (99 degrees C.), 220 degrees F. (104 degrees C.), 240 degrees F. (116degrees C.), 250 degrees F. (121 degrees C.) and 260 degrees F. (127degrees C.) and were strained by calendering rollers at thosetemperatures. The samples were tested for brittleness and ductilityusing a three-point flexural test as described in Example 1. The resultsare summarized in FIG. 4.

The results show that the samples strained at surface temperatures of210 and 220 degrees F. exhibited the highest levels of ductility, andthe samples strained at lower or higher surface temperatures exhibitedvarious degrees of brittle behaviour.

Without wishing to be bound by any particular scientific theory for thebehaviour demonstrated in this example, it is believed that in thesamples where the composite material is strained at surface temperaturesof 240, 250 or 260 degrees F., the majority of solidification of thepolyethylene occurs after straining, permitting comparatively largeinternal stresses to develop and resulting in brittle behaviour. In thesamples where the composite material is strained at a surfacetemperature of 190 or 200 degrees F., the majority of solidification ofthe polyethylene has occurred prior to straining and there isinsufficient polyethylene in melted form to permit enough movementwithin the composite matrix of the limestone and polyethylene to relieveinternal stresses by straining. Consequently, these samples also exhibitbrittle behaviour. When the surface temperature of a quarter-inch thicksheet of the invention is approximately 220 degrees F. (104 degrees C.),the temperature inside the sheet is approximately 225-245 degrees F.

Example 5

The effect of small concentrations of mica on the ductility of thecomposite material was studied. Compositions were prepared comprising78% limestone, 18% polyethylene (13.5% by weight recycled HDPE and 4.5%by weight virgin LLDE) and 2% by weight of lubricants, (zinc stearate orstearic acid), and either with no mica or having 0.2% by weight mica.The compositions were formed into sheets by the method described inExample 1 and were either strained by rolling at 210 degrees F. (99degrees C.) or were not strained. Sample A had no mica and was notstrained. Sample B had no mica and was strained. Sample C had mica andwas not strained. Sample D had mica and was strained. The four sampleswere tested for brittleness and ductility according to the methoddescribed in Example 1. The results are summarized in FIG. 5.

Sample A shows brittle behaviour and Sample C somewhat ductilebehaviour. While the introduction of mica is seen to improve ductilityof a composite that has not been rolled at 210 degrees F., thisimprovement in ductility is not considered sufficient for buildingapplications. Specifically, this improvement in ductility is notsufficient to permit the nailing of the composite without a highprobability of splitting due to its limited ductility. For example, whenSamples A and C were nailed approximately one-half inch from an edge,the samples split.

Samples B and D show the ductile behaviour of the composite after it hasbeen rolled at 210 degrees F. While Samples B and D appear on the samegraph, when the samples were repeatedly bent back and forth by handafter the flexural test, Sample B (no mica) broke or failed much earlierthan Sample D (with mica), indicating the enhanced ductility of SampleD. This improved ductility is of value in applications where enhancedtoughness is desired.

Example 6

The compressive and tensile properties of strained and unstrainedsamples were studied. Sheet materials were made as described in Example1, one sample of which was strained at 210 degrees F. (Sample A) and oneof which was not strained (Sample B). The compressive and tensileproperties of the samples were tested. The results are summarized inFIG. 6.

To determine the compressive properties, specimens of Sample A andSample B measuring approximately ¼inch by ¼inch were tested. For SampleA specimens, the ½ inch dimension was longitudinal to the direction ofstrain. The specimens were loaded with a compressive force toapproximately 100 pounds along the ½ inch dimension and load anddisplacement measured and recorded. At 100 pounds load, Sample Aexhibited approximately four times more compressive displacement thanSample B as evidenced the negative displacement shown in the lower leftquadrant of FIG. 6.

To determine the tensile properties, 4½ inch long dog-bone shapedspecimens with the middle section of the specimen approximately ¼inch by¼inch and 2 inches long were tested. For Sample A specimens, the 2 inchdimension was longitudinal to the direction of strain. The specimenswere tested with a tensile load applied along the 2 inch dimension andload and displacement measured and recorded. Sample A exhibited ductilebehavior and Sample B exhibited brittle behavior as evidenced by theresults shown in the upper right quadrant of FIG. 6.

The test results provided further evidence that ductility is increasedby straining the composite at approximately 210 degrees F.

Example 7

The effect of straining temperatures on sheet materials in which theresin comprises polypropylene were studied. Samples were prepared havinga composition comprising about 80% by weight limestone of 100 mesh, 18%by weight polypropylene and 2% by weight of lubricants (zinc stearate orstearic acid). They were kneaded and formed into sheets at about 300-325degrees F. (149-163 degrees C.). They were allowed to cool to respectivesurface temperatures of 210 degrees F. (99 degrees C.), 220 degrees F.(104 degrees C.), 230 degrees F. (110 degrees C.), 240 degrees F. (116degrees C.) and 250 degrees F. (121 degrees C.), and were strainedthrough calendering rollers at those temperatures. One sample was simplyallowed to cool to room temperature and was not strained. The sampleswere tested for brittleness and ductility using a three-point flexuraltest as described in Example 1. The results are summarized in FIG. 7.

Although the invention has been described in terms of variousembodiments, it is not intended that the invention be limited to thoseembodiments. Various modifications within the scope of the inventionwill be apparent to those skilled in the art. The scope of the inventionis defined by the claims that follow.

1. A method of making a sheet of building material for roofing or sidingapplications, comprising the steps of: (a) preparing a mixturecomprising a thermoplastic resin and a mineral filler, said resincomprising about 10% to 40% by weight of said mixture and said fillercomprising about 60% to 90% by weight of said mixture, said resincomprising polyethylene; (b) mixing said mixture at a temperature abovethe melting range of said resin; (c) forming said mixture into a sheet,said forming step being done at a temperature above the melting range ofsaid resin; (d) allowing a surface of said sheet to cool to atemperature below the midpoint of the melting range of said resin, saidtemperature being in the range of 205 to 225 degrees F.; and (e)straining said sheet by calendering, rolling or pressing said sheet whensaid surface temperature is within said temperature range of step (d) toincrease a length of said sheet by 1% to 10%.
 2. A method according toclaim 1, wherein said polyethylene is a mixture of high densitypolyethylene and linear low density polyethylene.
 3. A method accordingto claim 2, wherein the weight ratio of said high density polyethyleneto said linear low density polyethylene is about 3:1.
 4. A methodaccording to claim 1, further comprising lowering said surfacetemperature during said step of straining.
 5. A method according toclaim 1, wherein said filler is limestone.
 6. A method according toclaim 5, wherein said limestone is about 100 mesh in particle size.
 7. Amethod according to claim 1, wherein said filler is one of dolomite,talc, silica and flyash.
 8. A method according to claim 1, wherein saidthermoplastic resin comprises 10% to 35% by weight of said mixture.
 9. Amethod according to claim 1, wherein said resin comprises 10% to 34% byweight of said mixture and said filler comprises 66% to 90% by weight ofsaid mixture.
 10. A method according to claim 1, wherein said mixturefurther comprises mica.
 11. A method according to claim 10, wherein saidmica comprises 0.1% to 1.0% by weight of said mixture.
 12. A methodaccording to claim 1, wherein said mixture further comprises one or moreof carbon black, a UV stabilizer, a ductility enhancer and afireproofing component.
 13. A method according to claim 1, wherein saidstep of preparing said mixture comprises heating said filler and addingsaid resin to said filler.
 14. A method according to claim 10, whereinsaid filler used to prepare said mixture is at a temperature of about350 degrees F.
 15. A method according to claim 1, wherein saidtemperature of step (b) is in the range of 300 to 325 degrees F.
 16. Amethod according to claim 1, wherein said step of mixing is done in anopen atmosphere.
 17. A method according to claim 1, wherein said step ofstraining comprises passing said sheet between calendering rollers. 18.A method according to claim 17, wherein a calendering force applied bysaid calendering rollers is varied during the step of straining so as tocause a variation in color tone of said sheet.
 19. A method according toclaim 1, further comprising the step, after step (e), of applying aprotective coating to said surface of said sheet.
 20. A method accordingto claim 1, wherein the temperature in step (d) is in the range of 205to 220 degrees F.
 21. A method according to claim 1, wherein the sheetproduced in step (e) has a thickness of about one-quarter inch.